1. Field of the Invention
The present invention relates to a developing device and an image forming apparatus, and more specifically relates to an electrophotographic developing device that maintains a stable electrical charge for long-term use and stably provides high-quality images with high density and no fogging, and an electrophotographic image forming apparatus including the electrophotographic developing device.
2. Description of Related Art
In a typical electrophotographic image forming apparatus used in offices, a two-component developer is usually used. A two-component developer is used in a developing device in such a way that a toner and a carrier are agitated and mixed to electrically charge the toner and the charged toner is then provided on the surface of a photoreceptor to visualize (develop) an electrostatic latent image. Thus, only the toner is consumed in a repeated development of electrostatic latent images. The carrier is not consumed and remains in the developing device for a repeated use. In such a conventional method for development, only the consumed toner is replenished into a developing device.
A repeated image formation causes adhesion of toner particles, additives of the toner, and/or components of the toner matrix, which are generated because of mechanical stress in a developing device, on the surfaces of the carrier particles (this phenomenon is known as a “spent toner problem”). This lowers the electrical chargeability of the carrier, i.e., lowers the ability to electrically charge the toner. Thus, a repeated image formation impedes appropriate electrical charging of the toner, causing fogging and low image density of the images, which requires replacement of the developer after the formation of a certain number of images.
The development characteristics of a developer significantly differ between immediately before and after the replacement. Thus, the characteristics of the images also differ largely. Given this, Japanese Patent Application Laid-Open Publications Nos. Sho59-10071 and 2007-079578 each propose a developing device that achieves stable image quality through the prevention of degradation of the developer, the extension of the developer life, and the control of the development characteristics of the developer within certain ranges. In detail, the developing device maintains the charging characteristics of a developer in a developing device over an extended period through a repeated disposal of a small amount of degraded developer having lowered electrical chargeability and a repeated replenishment of a fresh developer, stabilizes the image characteristics, extends the developer life, and lowers the replacement frequency of the developer. Such a development system is known as an auto-refining development system or a trickle development system.
In such a system, a small amount of degraded developer is repeatedly discarded parallel to a repeated formation of images while a fresh developer having high electrical chargeability is repeatedly replenished. In this way, stable electrical chargeability of a developer in a developing device can be maintained for an extended period. As a result, high-quality images with high density and no fogging can be stably produced over an extended period.
Japanese Patent Application Laid-Open Publications Nos. 2012-63571 and 2009-205149 each disclose a developer using a porous ferrite carrier having a small specific weight for preventing carrier degradation caused by agitation stress.
A carrier having a smaller specific weight causes less agitation stress, and thus can suppress carrier degradation. However, a charge of a toner rises more slowly in using a carrier having a small specific weight than in using a carrier having a large specific weight. When an image with a high coverage rate (i.e., an image with a large black area) is continuously printed on a large amount of sheets, a large amount of toner is used. Upon replenishment of a toner, the charge amount of the toner is small. Electrical chargeability of a carrier having a small specific weight is therefore insufficient to increase the charge amount of a toner to a desired level, especially in a high-speed apparatus. When the charge amount of a toner is small, fogging is caused in an image and thus image quality is deteriorated.
A porous carrier having a small specific weight can suppress the degradation by virtue of its small specific weight, but has a small electrical chargeability due to its small specific weight. Thus, although an auto-refining development system is employed, the use of a porous carrier having a small specific weight cannot sufficiently achieve these advantages over an extended period.